Aldehydes are the usual starting materials for obtaining carboxylic acids. Their preferential position for this field of use is due to their varied availability and the oxidative conversion of the carbonyl group to the carboxyl group. In the context of processes practiced industrially, aldehydes are converted to carboxylic acids predominantly in the presence of catalysts. However, there are also known processes in which the use of catalysts is dispensed with. To avoid side reactions, minimum temperatures are employed both in the catalytic and in the noncatalytic processes; in general, a reaction temperature of 100° C. is not exceeded. Useful catalysts include predominantly salts of transition metals, especially salts of cobalt and of manganese, and of chromium, iron, copper, nickel, silver and vanadium. Frequently, carboxylic acid formation from aldehydes, even when observing optimal temperature conditions, is associated with side reactions and degradation reactions. This is equally true for reactions in the presence and in the absence of catalysts. In such cases, the selectivity of the conversion can be improved considerably by addition of alkali metal salts of weak acids to the reactants. However, a disadvantage in this process variant is that the salts have inhibiting action, and so the reaction time has to be prolonged for a full conversion of the starting materials. The oxidation of aliphatic aldehydes to the corresponding carboxylic acids using oxygen is a process which has been operated on the industrial scale for many years. Important aliphatic carboxylic acids which are placed by this process are the isomeric butyric acids, the isomeric pentanoic acids, 2-ethylhexanoic acid, n-heptanoic acid, n-nonanoic acid, and isononanoic acid based on 3,5,5-trimethylhexanoic acid (Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, 6th edition, volume 6, chapter “Carboxylic Acids, Aliphatic”; K. Weissermel, H.-J. Arpe, Industrielle Organische Chemie [Industrial organic chemistry], VCH Verlagsgesellschaft, 3rd edition, 1988, chapter “Oxo-Carbonsauren” [Oxo carboxylic acids]).
In a conventional embodiment of the oxidation of aliphatic aldehydes, the aldehyde is initially charged in a suitable reactor, for example in a tubular reactor provided with a baffle plate and optionally also containing random packing, and the oxygen or the oxygen-containing gas mixture is passed through the aldehyde from the bottom. Such a procedure using a bubble column is described, for example, in DE 100 107 69 C1, DE 100 107 70 C1 and DE 100 107 71 C1. The reaction of the liquid aliphatic aldehyde with gaseous oxygen is performed generally at temperatures of preferably 40 to 80° C. and at standard pressure. Higher reaction temperatures and pressures are possible, but the reaction temperature selected should not exceed 100° C. and the pressure should not be higher than 1 MPa. Both noncatalytic processes (DE 100 107 69 C1, DE 100 107 69 C1) and transition metal-catalyzed processes (DE 100 107 71 C1; G. R. Lappin, J. G. Sauer “Alpha Olefins Applications Handbook”, Marcel Dekker Inc., New York and Basel, 1989, Chapter 11, “Fatty Acids”) are known.
In a further embodiment, the reactor used is a trickle tower containing shaped bodies. The aldehyde is allowed to trickle downward through the packing, and oxygen or an oxygen-containing gas mixture is introduced simultaneously into the tower in cocurrent or counter-current. In addition, the prior art also describes the use of stirred tanks in aldehyde oxidation with gaseous oxygen (WO2001/46111 A1).
The industrial oxidation of aliphatic aldehydes with gaseous oxygen in high-capacity conventional reactors is constantly being optimized and is now being implemented at high conversion with good carboxylic acid selectivities. Disadvantages, however, are the comparatively long residence times of the reaction mixture, which may be up to several hours and impair the economic viability of the oxidation process. In the case of long residence times in the oxidation reactors, increased by-product formation is additionally to be expected, for example with cleavage products such as lower carboxylic acids, carbon monoxide or carbon dioxide, and so the selectivity of the oxidation reaction can suffer in spite of the good values which have now been achieved. The handling of aliphatic aldehydes and oxygen in high-capacity reactors likewise entails a high level of capital costs and control complexity for safety devices. Due to the exothermic oxidation reaction, it also has to be ensured that the heat of reaction released can be removed in a rapid and controlled manner in order to avoid an unwanted temperature rise. Therefore, cooling devices of appropriate dimensions should be installed. For lowering of the residence time, working at elevated pressures can be contemplated, but the employment thereof in the oxygen oxidation of aliphatic aldehydes in high-capacity reactors can become very problematic for safety reasons and entails high capital costs. Aldehyde oxidations implemented industrially are therefore usually conducted at standard pressure or if appropriate only slightly elevated pressure.
The constant advance in miniaturization in information and communications technology since the 1960s has also found its way into the field of chemistry since the 1980s, first in the laboratory and in the last few years also increasingly in production. While the miniaturization of reaction spaces is utilized in high-throughput studies to test a particular reaction in a small space in a parallel manner under a maximum number of different conditions in order, for example, to develop suitable catalysts in a controlled manner (WO 00/51720), miniaturization in microscale reaction technology serves for performance of a chemical reaction under the same conditions in a multitude of parallel reaction spaces in order to be able to prepare corresponding amounts of chemical products under defined conditions. The transition from the macroscopic process regime to microscale reaction technology offers the following advantages in terms of process technology: As a result of the small lateral dimensions, heat and mass transfer operations by thermal conduction and diffusion are greatly intensified and enable high performances for heat exchangers and mixers. In exothermic reactions, the heat of reaction can therefore be removed much more rapidly than in high-capacity conventional reactors. A characteristic feature of microscale reaction technology is the high ratio of surface to volume, i.e. the high specific surface area, such that surface-controlled phenomena too can also be distinctly intensified.
As a result of these intensifications, higher space-time yields, i.e. shorter residence times, are possible. The intensification potential of the micro-reactors is fully exploited especially when new reaction conditions which are unusual for conventional reactors or cannot be realized in such conventional reactors are established. Reference is made in this context to “novel process windows”, a term known, for example, from Energy Environ. Sci., 2008, 1, pages 467-478. Especially the establishment of higher pressures and temperatures is possible as a result. An additional factor is the safety aspect that these conditions frequently cannot be controlled in high-capacity conventional reactors and can be established safely only in microreactors. However, even without this intensification potential, i.e. without the use of unusually high pressures and temperatures, the micro-reactor can enable safer operation than with conventional technology.
Typical apparatus dimensions for microscale reaction technology are within a range from a few millimeters to a few micrometers. For the manufacture of microscale process technology components in these dimensions, suitable structuring methods are available, for example precision engineering processes such as microscale machining or microscale spark erosion, laser ablation processes, forming and molding processes or etching processes. In the case of dimensions below 100 nanometers in particular, reference is made to nanotechnology, whereas apparatus dimensions above a few millimeters pertain basically to milliscale apparatus. These are then followed by conventional reactors such as shell-and-tube apparatus, tubular reactors and finally stirred tanks.
An overview of the field of microscale process technology and of the manufacture of microscale process technology components can be found in Winnacker, Küchler, Chemische Technik [Chemical Technology], Wiley-VCH, 5th edition, 2004, volume 2, chapter 8 “Mikroverfahrenstechnik” [Microscale Process Technology], and in Microreactors in Organic Synthesis and Catalysis, Wiley-VCH, 2008, chapter 1 “Fabrication of Microreactors Made from Metals and Ceramics” and chapter 2 “Fabrication and Assembling of Microreactors Made from Glass and Silicon”.
Against the background of the above-outlined disadvantages possessed by the oxidation of aliphatic aldehydes with oxygen or oxygen-containing gases in conventional high-capacity reactors, it is an object of the present invention to provide a process for oxidation of aliphatic aldehydes to the corresponding carboxylic acids which, at high aldehyde conversion and high selectivities for the desired carboxylic acids, enables a lowering of the residence time. The associated increase in the space-time yield of the desired carboxylic acid increases the economic viability of the oxidation process. The process to be provided shall also be operated in a safer manner and feature lower capital costs for safety devices.